Rudyard Kipling called it the eighth Wonder of the World. It has been judged the world's top travel destination and is New Zealand's most famous tourist destination. Milford Sound (Piopiotahi in Maori) is a fjord in New Zealand's South Island, within Fiordland National Park and the Te Wahipounamu World Heritage site. It is named after Milford Haven in Wales, while the Cleddau River which flows into the sound is also named for its Welsh namesake. The Maori named the sound Piopiotahi after the, now extinct, thrush-like piopio bird. Piopiotahi means "a single piopio", harking back to the legend of Mâui trying to win immortality for mankind - when Maui died in the attempt, a piopio have flown here in mourning. Milford Sound runs 15 kilometres inland from the Tasman Sea, and is surrounded by sheer rock faces on either side. Among the peaks are The Elephant, said to resemble an elephant's head, and The Lion, in the shape of a crouching lion. Lush rain forests cling precariously to the cliffs, while seals, penguins, and dolphins frequent the waters and whales can be seen sometimes. There are two permanent waterfalls in MIlford Sound: Lady Bowen Falls and Stirling Falls. However, after heavy rain, hundreds of temporary waterfalls run down the steep sided rock faces that line the fjord. They are fed by rain water and will only last for a few days after the rain stops.

Milford Sound is known as the wettest inhabited place in New Zealand and one of the wettest in the world. Rainfall can reach 250 mm during a span of 24 hours. The rainfall creates dozens of temporary waterfalls which cascade down the cliff faces. Smaller falls may never reach the bottom of the sound, and they drift away in the wind. Accumulated rainwater can at times cause portions of the rain forest to lose its grip on the sheer cliff faces, resulting in tree avalanches into the sound. Milford Sound was initially overlooked by European explorers, because its narrow entry did not appear to lead into such large interior bays. Sailing ship captains such as James Cook, who bypassed Milford Sound on his journeys for just this reason, also feared venturing too close to the steep mountainsides, afraid that wind conditions would prevent escape. Thus the name of the Doubtful Sound, so named as Cook thought it doubtful he would escape if he sailed in).

The fjord was a playground for local Maori who had acquired a large amount of local marine knowledge including tidal patterns and fish feeding patterns over generations prior to european arrival. The fjord remained undiscovered by Europeans until Captain John Grono discovered it in around 1812 and named it Milford Haven after his homeland in Wales. While Fiordland remained one of the least-explored areas of New Zealand up to the 20th century, Milford Sound's natural beauty soon attracted international renown, and led to the discovery of the Mackinnon Pass in 1888, soon to become a part of the new Milford Track, an early walking tourism trail. Even with its remote location and the long journey from the nearest population centers, the beauty of the Milford Sound landscape draws thousands of visitors each day. An underwater tourist observatory found in one of the bays of the sound provides viewing of black coral, usually only found in much deeper waters. A dark surface layer of fresh water, stained by tannins from the surrounding forest, allows the corals to grow close to the surface here. In rainy and stormy days tourists can admire the play of the wind with the numerous waterfalls in Milford Sound. When meeting the cliff face the powerful wind often goes upward and waterfalls with a vertical drop get caught by wind, causing the water to go upwards.